Hands-free kits are the best way of protecting mobile phone users against potentially harmful radiation, though shield devices do have some effect, experts say.

A report published on Friday looked at four types of shields that can be used, but its author said hands-free devices - which had been examined in a previous study - still came out on top.

The devices looked at in the report, published by the Department of Trade and Industry (DTI) were:

shields which cover the whole phone

ear pieces which cover earphones

buttons which can be placed on the phone

antennae clips

The report says that shield covers and antennae clips work the best, but reduce the phones power and coverage.

You can buy shielding devices which do reduce the dose of radiation but many of them reduce the effectiveness of the phone

Dr Michael Manning, report author

It does not name individual brands.

The report is published two years after an expert panel chaired by Sir William Stewart, former chief scientific advisor called for independent testing of such devices.

The report's author Dr Michael Manning, who has been carrying out research in the area for several years, told BBC News Online: "The main finding of this report is that you can buy shielding devices which do reduce the dose of radiation but many of them reduce the effectiveness of the phone.

"If the user is looking to reduce the exposure from a mobile phone, they would do well to consider the option of a hands-free kit, rather than using shield devices."

Hands-free kits were examined in a previous study by Dr Manning.

Reduced power

Of the shielding devices, which all use absorbent material, tested in this latest report, the sleeves which shield the whole phone came out best.

However, they also reduced the effectiveness of the phone, by weakening signal reception.

The shield covers were one of the devices examined

Dr Manning admitted that in some instances, the phone may automatically boost its power to compensate for the weak signal, thereby negating the safety effects of the cover.

Clips which can be put on top of the phone's antennae also work, but again reduce effectiveness, the report says.

Buttons, which can be placed on various parts of the phone to absorb radiation do "something, but not much", said Dr Manning.

Least effective are earpiece shields, which do "very little".

He said which device people used was a matter of convenience for them.

Exposure

Simon Rockman of What Mobile magazine said: "Because they've got a shield, they feel they've got a solution to a perceived problem. That's what makes them better.

"It's a placebo effect. It's not scientific."

A spokesman for the Federation of Electronic Industries, speaking prior to the report's publication, said the balance of evidence to date suggested that mobile phones did not cause any adverse health effects.

He also pointed out that information on SAR values, the amount of energy put into your body by the phone which varies between types, had been available since October last year.

He added that the World Health Organization said that scientific evidence did not indicate the need for protective devices on mobile phones.

"The WHO has said that if people are still concerned they can make a personal choice to reduce their exposure to radiation waves by using a hands-free kit."

He said manufacturers recommended people use the hands-free kit designed for their particular phone.